It is well known that metallocene catalysts and other so called “single site catalysts” incorporate comonomer more evenly than traditional Ziegler-Natta catalysts when used for catalytic ethylene copolymerization with alpha olefins. This fact is often demonstrated by measuring the composition distribution breadth index (CDBI) for corresponding ethylene copolymers. The definition of composition distribution breadth index (CDBI) can be found in U.S. Pat. No. 5,206,075 and PCT publication WO 93/03093. The CDBI is conveniently determined using techniques which isolate polymer fractions based on their solubility (and hence their comonomer content). For example, temperature rising elution fractionation (TREF) as described by Wild et al. J. Poly. Sci., Poly. Phys. Ed. Vol. 20, p 441, 1982 can be employed. From the weight fraction versus composition distribution curve, the CDBI is determined by establishing the weight percentage of a copolymer sample that has a comonomer content within 50% of the median comonomer content on each side of the median.
Generally, Ziegler-Natta catalysts produce ethylene copolymers with a CDBI of less than about 50%, consistent with a heterogeneously branched copolymer. Typically, a plurality of prominent peaks is observed for such polymers in a TREF (temperature raising elution fractionation) analysis. Such peaks are consistent with the presence of heterogeneously branched material which generally includes a highly branched fraction, a medium branched fraction and a higher density fraction having little or no short chain branching. In contrast, metallocenes and other single site catalysts will most often produce ethylene copolymers having a CDBI of greater than about 65% and which usually contain a single prominent peak in a TREF analysis, consistent with a homogeneously branched copolymer.
Despite the forgoing, methods have been developed to access polyethylene copolymer compositions having a broadened comonomer distribution (i.e. more Ziegler-Natta like) while otherwise maintaining product characteristics typical of metallocene and single site catalyst resin, such as high dart impact strength for blown film. Such resins can be made, for example, by using a mixture of metallocene catalysts in a single reactor or by blending metallocene produced ethylene copolymers.
U.S. Pat. Nos. 5,382,630 and 5,382,631 describe blend compositions having a narrow molecular weight distribution, but a bimodal comonomer distribution. The blends are made using two metallocene produced resins of approximately the same molecular weight, but having different comonomer contents.
A mixed catalyst system containing a “poor comonomer incorporator” and a “good comonomer incorporator” is disclosed in U.S. Pat. Nos. 6,828,394 and 7,141,632. The poor comonomer incorporating catalyst may be a metallocene having at least one fused ring cyclopentadienyl ligand, such as an indenyl ligand, with appropriate substitution (e.g. alkyl substitution at the 1-position). The good comonomer incorporating catalyst was selected from an array of well known metallocenes and which were generally less sterically encumbered toward the front end of the molecule than the poor comonomer incorporator. These mixed catalyst systems produced polyethylene copolymers having a bimodal TREF distribution in which two elution peaks are well separated from one another, consistent with the presence of higher and lower density components. The mixed catalysts also produced ethylene copolymer having a broadened molecular weight distribution relative to ethylene copolymer made with either one of the single metallocene component catalysts.
U.S. Pat. No. 7,572,875 also describes the use of a mixed catalyst system comprising two metallocene catalysts. Each catalyst component is supported on a chemically modified support to produce ethylene copolymers, which when made into film, have high dart impact values. Polymerization takes place in a continuous slurry phase polymerization process using a loop reactor. In an embodiment of the invention, a hafnocene catalyst is combined with a zirconocene catalyst. The polymers made are further characterized as having a so called rheology “breadth parameter a” (i.e. a Carreau-Yasuda, CY parameter) of from 0.45 to 0.7.
U.S. Pat. Nos. 6,248,845; 6,528,597 and 7,381,783 disclose that a bulky ligand metallocene based on hafnium and a small amount of zirconium can be used to provide an ethylene/1-hexene copolymer which has a bimodal TREF profile. It is taught that the hafnium chloride precursor compounds used to synthesize the bulky metallocene catalysts are either contaminated with small amount of zirconium chloride or that zirconium chloride may be deliberately added. The amounts of zirconium chloride present range from 0.1 mol % to 5 mol %. Hence, the final hafnocene catalysts contain small amounts (i.e. 0.1 to 5 mol %) of their zirconocene analogues. Since zirconium based catalysts are well known to have superior activity relative to their hafnium analogs it is reasonable to expect that the products made have a significant contribution from the zirconocene species. If this is the case, then it is perhaps not surprising that a bimodal TREF profile results.
U.S. Pat. Nos. 6,956,088; 6,936,675 and 7,179,876 disclose that use of a “substantially single” bulky ligand hafnium catalyst provides an ethylene copolymer composition having a CDBI of below 55%, especially below 45% as determined by CRYSTAF. Recall, that hafnocene catalysts derived from hafnium chloride are expected to have zirconocene contaminants present in low amounts. U.S. Pat. Nos. 6,936,675 and 7,179,876 further teach that the CDBI could be changed under different temperature conditions when using hafnocene catalysts. Polymerization at lower temperatures gave ethylene copolymer having a broader composition distribution breadth index relative to polymers obtained at higher temperatures. For example, use of the catalysts bis(n-propylcyclopentadienyl)hafnium dichloride or bis(n-propylcyclopentadienyl)hafnium difluoride in a gas phase reactor for the copolymerization of ethylene and 1-hexene at ≦80° C., gave copolymers having a CDBI of between 20 and 35%, compared to CDBI values of between 40 and 50% for copolymers obtained at 85° C.
Examples where a single metallocene catalyst in a single gas phase reactor produces ethylene copolymers having very narrow molecular weight but a broadened composition distribution and multimodal TREF profile are rare.
U.S. Pat. No. 6,932,592 describes very low density (i.e. <0.916 g/cc) ethylene copolymers produced with a bulky non-bridged bis-Cp metallocene catalyst. A preferred metallocene is bis(1-methyl-3-n-butylcyclopentadienyl)zirconium dichloride. The examples show that in the gas phase, supported versions of this catalyst produce copolymer from ethylene and 1-hexene which has a CDBI of between 60 and 70% and a bimodal comonomer distribution as measured by temperature raising elution fractionation (TREF). A bimodal TREF profile is more likely to be observed for lower density copolymeric materials than for materials having a density above 0.916 g/cc due to the fact that any “higher density” fraction which may be present would be further separated from a polymer fraction having a relatively high amount of comonomer.
Use of phosphinimine catalysts for gas phase olefin polymerization is the subject matter of U.S. Pat. No. 5,965,677. The phosphinimine catalyst is an organometallic compound having a phosphinimine ligand, a cyclopentadienyl type ligand and two activatable ligands, and which is supported on a suitable particulate support such as silica. The exemplified catalysts had the formula CpTi(N═P(tBu)3)X2 where X was Cl, Me or Cl and —O-(2,6-iPr-C6H3).
We now disclose that a single phosphinimine catalyst having appropriate substitution on a cyclopentadienyl ligand can, under appropriate conditions in a single gas phase reactor, provide a linear low density polyethylene material having i) a very narrow molecular weight distribution ii) a CDBI of between 50 and 66 wt %, iii) a bimodal TREF profile, and iv) a density of from 0.916 g/cc to 0.920 g/cc. Such materials are difficult to access without use of a mixed metallocene catalyst system, use of multiple reactors systems, or use of post reactor blending methods. Although, the phosphinimine catalyst structures employed herein have been previously disclosed in the patent literature, their use in making the novel film compositions of the present invention has not. For reference see:                i) U.S. Patent Application No. 2008/0045406 expressly illustrates the use of supported (1,2-(n-butyl)(C6F5)Cp)Ti(N═P(t-Bu)3)Cl2 to copolymerize ethylene and 1-hexene in the gas phase in a bench scale reactor. The catalyst was activated with an ionic activator having an active proton. The polymer composition details provided for each run included information on branch content, molecular weight and molecular weight distribution. There is no discussion of film preparation or film properties.        ii) U.S. Pat. No. 7,531,602 expressly illustrates the use of (1,2-(n-propyl)(C6F5)Cp)Ti(N═P(t-Bu)3)Cl2 in a gas phase polymerization of ethylene at the bench scale. The phosphinimine catalyst is activated with methylaluminoxane and is supported on silica. There is no discussion of films derived from the polymer obtained using this particular phosphinimine catalyst. Instead, the patent is directed to blends in which the instant polymer is used as the higher molecular weight component in a bimodal composition.        iii) U.S. Pat. No. 7,064,096 discloses the use of (1,2-(n-butyl)(C6F5)Cp)Ti(N═P(t-Bu)3)Cl2, but only in the context of a dual catalyst formulation in which the phosphinimine catalyst is co-supported with a phenoxyimine catalyst on a silica support. The patent is directed to the formation of bimodal resins suitable for application in pipe.        iv) U.S. Pat. Nos. 7,323,523; 7,321,015 and U.S. Patent Application No. 2008/0108763 each disclose the use of (1,2-(n-butyl)(C6F5)Cp)Ti(N═P(t-Bu)3)Cl2 and (1,2-(n-hexyl)(C6F5)Cp)Ti(N═P(t-Bu)3)Cl2 but only in the context of a dual catalyst formulation in which the phosphinimine catalyst is co-supported with a phenoxyimine catalyst on a silica support. These disclosures are directed to the formation of bimodal resins suitable for application in pipe.        